The testing of integrated circuits and of their interconnections in packaging is an important task in the manufacturing of electronic and data processing equipment. It is particularly important that the testing methods are fast and do not require much preparatory operations for their execution.
In principle, testing can be done by applying power and data signals to circuitry and extracting resulting data signals and electrical conditions from circuitry through the regular connection pins or through mechanical contact probes. Such testing is, however, slow and not very effective because of the limited number of connections which can be made. It is therefore not well suited for highly integrated circuits and dense packaging which are used today.
Therefore, some methods of contactless testing have been recently suggested using either electron beam or laser technology. E-beam testing was described in an article by E. Menzel et al.: "Fundamentals of Electron Beam Testing of Integrated Circuits", published in Scanning, Vol. 5 (1983), pp. 103-122. The E-beam is used as a contactless probe, either in a passive or active mode. However, the utilization of electron beams for testing has certain disadvantages and limitations. Scanning or flooding a substrate by an E-beam leads also to the charging of the substrate because the incident electrons have energies much larger than the work function of the substrate, i.e. they produce secondary electrons. This strongly influences the voltage contrast between different points on a substrate and hinders the measurements. Furthermore, reflected high energy electrons strongly complicate simultaneous measurement for many different points using position-sensitive detectors because these high energy electrons must be separated from the lower energy electrons which carry the required information.
More recently, the use of an optical beam, e.g. a laser beam, for integrated circuit testing has been suggested. The photons of a laser beam can excite electron emission from the target, and the laser beam can thus be used for contactless testing of electronic circuitry.
European Patent Application No. 0,205,760 (U.S. counterpart issued as U.S. Pat No. 4,670,710) entitled "Noncontact Testing of Integrated Circuits" discloses a technique of directing a laser beam in a scanning motion to lines of test points so that photoelectrons are generated in dependance of the operating conditions of those test points. The generated electrons are then directed to a two-dimensional array electron detector so that an analysis can be made.
In European Patent Application No. 0,216,077 (U.S. counterpart application Ser. No. 778,823 filed Sept. 23, 1985) a testing procedure is described in which laser light is directed to the entire surface of a circuit chip, and in which that laser light causes generation of photoelectrons in dependance of the voltage present at each point. The electrons generated are directed either to a luminescent target whose image is then evaluated, or the electrons are directed to channel plates and further to a luminescent target and an optical processing system.
In both systems disclosed in the two above-mentioned European patent applications, power and test data signals have to be applied to the tested circuits through normal pins and chip connections, to bring them into an operating status that is to be detected in the testing procedure. This is a limitation of the testing possibilities.